Why are impact craters always
round? Most incoming objects must strike at some angle from vertical, so why
don't the majority of impact sites have elongated, teardrop
shapes?

"When geologists and astronomers first recognized that lunar and terrestrial
craters were produced by impacts, they surmised that much of the impacting body
might be found still buried beneath the surface of the crater floor. (Much wasted
effort was expended to locate a huge, buried nickel-iron meteorite believed to
rest under the famous Barringer meteor
crater near Winslow, Ariz.) Much later, however, scientists realized that at
typical solar system velocities--several to tens of kilometers per second--any
impacting body must be completely vaporized when it hits.

"At the moment an asteroid collides with a planet, there is an explosive
release of the asteroid's huge kinetic energy. The energy is very abruptly
deposited at what amounts to a single point in the planet's crust. This sudden,
focused release resembles more than anything else the detonation of an extremely
powerful bomb. As in the case of a bomb explosion, the shape of the resulting
crater is round: ejecta is thrown equally in all directions regardless of the
direction from which the bomb may have arrived.

"This behavior may seem at odds with our daily experience of throwing rocks
into a sandbox or mud, because in those cases the shape and size of the 'crater'
is dominated by the physical dimensions of the rigid impactor. In the case of
astronomical impacts, though, the physical shape and direction of approach of the
meteorite is insignificant compared with the tremendous kinetic energy that it
carries.

"An exception to this rule occurs only if the impact occurs at an extremely
shallow, grazing angle. If the angle of impact is quite close to horizontal, the
bottom, middle and top parts of the impacting asteroid will strike the surface at
separate points spread out along a line. In this case, instead of the energy
being deposited at a point, it will be released in an elongated zone--as if our
'bomb' had the shape of a long rod.

"Hence, a crater will end up having an elongated or elliptical appearance only
if the angle of impact is so shallow that different parts of the impactor strike
the surface over a range of distances that is appreciable in comparison with the
final size of the crater as a whole. Because the final crater may be as much as
100 times greater than the diameter of the impactor, this requires an impact at
an angle of no more than a few degrees from horizontal. For this reason, the vast
majority of impacts produce round or nearly round craters, just as is observed.

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